1. Field of the Invention
The present invention relates to the field of semiconductor integrated circuit (IC) manufacturing, and more specifically, to a phase-shifting mask for extreme ultraviolet (EUV) photolithography and a method of forming a phase-shifting mask for EUV photolithography.
2. Discussion of Related Art
In 1965, Gordon Moore first observed that the number of devices per area on an integrated circuit (IC) chip doubled approximately every 18 months. For over 3 decades, the semiconductor industry has managed to achieve the rate of increase in device density projected by Moore's Law by developing many new designs and new processes. Significant improvements in doping, deposition, and etch have enhanced the precision that concentration, depth, and thickness can be controlled across the IC chip.
In the past, scaling of the chip has been accomplished by shrinking the dimensions of the devices in the substrate as well as the dimensions of the interconnections between the devices. Thus, a continual enhancement in photolithography has contributed to repeated reductions in a critical dimension (CD) that can be successfully patterned in a feature on a device.
According to the Rayleigh criterion, the minimum CD which can be resolved by an imaging system is directly proportional to a wavelength of the radiation, or light, from the illumination source and inversely proportional to a numerical aperture (NA) of the projection optics. The NA is the product of a refractive index (n) and the sine of the convergence angle (theta).
The wavelength of light used for exposure of photoresist depends on the available illumination source and has been decreased over time from 436 nanometers (nm) to 365 nm (both being ultraviolet or UV light), and, subsequently, to 248 nm and then to 193 nm (both being deep ultraviolet or DUV light). The NA of the projection optics has been steadily increased up to about 0.85.
However, as device dimensions continue to shrink, the fundamental limitations of optics will play increasingly larger roles. In particular, diffraction will degrade an aerial image produced by the imaging system when the CD becomes smaller than the exposure, or actinic, wavelength. Consequently, wavefront engineering using resolution enhancement techniques (RETs) may be needed to achieve a sufficiently wide process latitude in the sub-actinic regime.
A phase-shifting mask (PSM) is a type of RET. Unlike a conventional binary mask that only modulates amplitude of light, a PSM also modulates phase of light to use interference to mitigate the detrimental effects of diffraction and enhance resolution of the optics.
In the future, the exposure wavelength will be decreased to even shorter wavelengths, including extreme ultraviolet or EUV light. However, the EUV optics have lower NA than the DUV optics that are currently in use so RETs may have to be employed.
Thus, what is needed is a phase-shifting mask for EUV photolithography and a method of forming such a phase-shifting mask for EUV photolithography.